In recent years, most implantable pacemakers have included the capability of transmitting an intracardiac electrical signal (e.g., P-waves, R-waves, etc.), either alone or in combination with marker signals. The intracardiac electrical signal provides data indicative of heart activity, including the contraction of the atria as sensed by the pacemaker sensing circuits, the contraction of the ventricles as also sensed by the pacemaker sensing circuits, and the timing therebetween. Further, if stimulation pulses are generated by the pacemaker in order to cause a particular heart chamber to contract at a particular time, such is also evident in the intracardiac electrical data telemetered from the pacemaker. As used herein, the intracardiac electrical data includes the intracardiac electrical signals and any marker data.
The display or printing of the intracardiac electrical signal as a function of time is known as an intracardiac electrogram, EGM or IEGM. Advantageously, the intracardiac EGM provides a "picture" of the performance of the heart and pacemaker. Any problems associated with the heart, or with the pacemaker, are usually evident from an analysis of the intracardiac EGM. If intracardiac electrical data is not available, then a conventional surface electrocardiographic signal may be made using, e.g., skin electrodes. As used herein, display or printing of the surface electrocardiographic signals as a function of time is known as a surface electrocardiogram, or ECG. The surface ECG also provides a "picture" of the performance of the heart and a pacemaker.
Because special equipment must be used to record a surface ECG, it is generally preferred for a patient already having an implanted pacemaker to utilize the intracardiac electrical data from the pacemaker as the primary indicator of the heart's performance. To this end, some pacemaker manufacturers include as an integral part of their diagnostic and programming devices (used by the physician or cardiologist to program and interrogate the implanted pacemaker), the ability to print and/or display the intracardiac electrical data received from the pacemaker as a function of time.
Such intracardiac electrical data, when printed or displayed as a function of time, appears dissimilar to a surface ECG. (It is noted that the shape of some of the waveforms included within the intracardiac electrical data, e.g., a P-wave, representing contraction of the atria, or an R-wave, representing contraction of the ventricles, may appear somewhat distinct from the shape of corresponding waveforms included within a surface electrocardiographic signal due to the different location from which such signals are sensed, one being sensed from inside the heart, the other being sensed at the skin of the patient. However, the timing relationship between such waveforms remains approximately the same.)
Further, some recent diagnostic/programming devices (frequently referred to as "programmers") include the ability to "freeze" the incoming intracardiac electrical data so that the displayed intracardiac EGM may be carefully studied. An example of a diagnostic/programming device which includes the capability of displaying, printing and freezing the incoming intracardiac electrical data is described in applicant's earlier U.S. Patent No. 4,809,697, to Causey, III et al., which patent is hereby incorporated herein by reference. It should also be noted that the diagnostic/programming device described in the referenced patent also includes the capability of selectively displaying the surface electrocardiographic signal as well as intracardiac electrical data.
It is not uncommon when observing intracardiac electrical data or surface electrocardiographic signals on a programming device, or equivalent display, for "noise" to mask out important features of the waveform being studied. (As used herein, the term "noise" refers to any unwanted signal.) When this occurs, it is necessary to identify the source of the noise, if it is identifiable, and then make attempts to eliminate the source of the noise, or at least minimize its effect.
For example, there may be a 60 Hz component present in the waveform which originates with the line power. If this occurs, attempts must be made to filter out the 60 Hz component before it interacts with the circuits displaying the intracardiac electrical data or surface electrocardiographic signal. Once this is successfully done, the intracardiac electrical data or surface elecardiographic signal (in the absence of the 60 Hz background noise) may be retaken. Having to retake such data, however, is not only inconvenient and possibly stressful for the patient, but is also expensive to perform.
Unfortunately, some types of noise are not specific to a particular frequency, and may not originate from any one source. Hence, it is difficult to separate such noise from the incoming data without also compromising the integrity of the incoming data. In such instances, it is usually necessary to apply one or more different kinds of filters to the incoming data in an attempt to remove the noise without adversely impacting the integrity of the signal. Unfortunately, this typically requires a time-consuming iterative process wherein a particular filter type is selected prior to gathering the intracardiac electrical data, after which the data is gathered and evaluated.
Then, based on the results of the evaluation, another filter type is selected (or the prior filter type is modified), and additional data is gathered and evaluated. This process is repeated as many times as is necessary in order to optimize the intracardiac electrical data so that any cardiac phenomena manifested therein can best be detected and observed. Disadvantageously, such an iterative process is not only expensive (both in terms of time consumed, as well as in the costs associated with the design, fabrication and test of the various signal processing elements, e.g., filters which must be used), but it is also bothersome and stressful to the patient. What is needed, rather, is a system wherein the intracardiac electrical data or surface elecardiographic signal can be acquired once, and thereafter processed as many times as desired using inexpensive, flexible signal processing methods and techniques.
As is evident from the above description, one of the problems associated with monitoring the performance of implanted products through telemetry and/or surface patient connections is that the data is typically displayed in real-time. Because real-time data is only present for an instant of time, it is thus common to "capture" or "freeze" the data by printing it and/or by storing it in memory for subsequent display. However, as the real-time data is thus captured or frozen, a desired signal processing technique must be applied, e.g., filtering, in order to remove any unwanted signals or interference as the data is captured.
The use of such real-time signal processing techniques commonly introduces distortion which denies the accurate reproduction of the signal of interest. While many sophisticated signal processing circuits are known in the art which could be used to process a given signal in real-time for a particular purpose, such circuits are complex and expensive to make and operate. Further, if the intracardiac electrical data is to be processed in real-time for more than one purpose, it is typically necessary to successively apply newly acquired real-time data to an appropriate signal processing circuit. Hence, what is needed is a method for analyzing the acquired intracardiac electrical data in non-real-time, e.g., "off-line", thereby allowing a single acquisition of the data to serve multiple purposes.
An additional problem facing users of products which are regulated by the Federal Drug Administration (FDA), or any other governmental agency, such as pacemakers and programming devices used with pacemakers, is that the appropriate governmental agency must not only initially approve the device itself, but also must approve any modifications subsequently made to the devices, e.g, to any electronic circuitry used within such devices which interfaces in real-time with the patient as the intracardiac electrical data is obtained. Hence, a circuit modification, e.g., to change a filter in order to enhance the observability of cardiac phenomena manifested in the intracardiac electrical data, may require prior FDA approval, which approval may result in a significant delay and expense before it is obtained.
What is thus needed is a system wherein modifications to improve the observability of cardiac phenomena can be readily made in a way which does not affect the hardware or software utilized in real-time to initially acquire the intracardiac electrical data, and hence which does not need to go through an approval cycle. The present invention advantageously addresses the above and other needs.